Because it includes investments and so it is a better indicator of need than wage.
There are plenty of people who have small wages/salary, or even zero wages/salary, and instead rely on investment income.
For example, most landlords. Or retired people. Or the idle wealthy, like the various unemployed children of billionaires. Jeff Bezos has a salary of roughly $80K at Amazon. But he is way better off than someone with a salary of $90K.
If you look at wages instead of overall income, you might think some of those folks are struggling when they absolutely aren’t.